This study is being conducted to see whether four years of therapy in adult onset diabetics with the FDA approved Angiotensin II receptor antagonist, losartan (a new antihypertensive medication), will lead to better ultimate renal function than the use of placebo plus other blood pressure medications. Renal function will be measured by doubling of the serum creatinine, a common blood marker for kidney function, and by the development of end stage renal disease, the need for dialysis or transplantation. A secondary purpose is to evaluate whether this AII receptor antagonist, losartan, leads to less cardiovascular disease as measured by myocardial infarcts, bypass operations, and other invasive interventions. Other secondary objectives will deal with whether losartan reduces proteinuria versus placebo (plus other antihypertensives) and to assess the quality of life and healthcare utilization in the two groups. The study is a multicenter, randomized, double-blind, placebo controlled trial with two arms. Approximately 1520 patients who are adult onset diabetics with proteinuria and only mild to moderate reduction of thier renal function (elevated serum creatinine) will be randomized to receive either losartan or a placebo. The dose of each will be increased as tolerated by the blood pressure control. Other antihypertensives may be used in both arms of the study for blood pressure control. The primary endpoints of the study are doubling of the serum creatinine or reaching renal failure with the need for dialysis and transplantation.